Generally, photolithography is an essential process that is used in semiconductor device fabrication. Photolithograph is carried out in a manner of coating a photoresist layer on a wafer, carrying out exposure on the photoresist layer using a photomask having a prescribed layout, developing the exposed photoresist layer into a photoresist pattern corresponding to the prescribed layout.
As design rules are lowered according to the rising degree of semiconductor device integration, a defect is caused to a pattern by optical proximity effect (OPE) with a neighbor pattern in performing photolithography. Namely, in case of forming a rectangular pattern, corner rounding takes place by optical diffraction and interference to form rounded corners of the rectangular pattern. Moreover, a pattern size in a dense area having patterns formed densely is formed smaller than that in a sparse area where patterns are sparse. This is attributed to OPE as well. The OPE is explained with reference to the drawings as follows.
Referring to FIG. 1, in case of forming a plurality of contact holes having the same diameter by etching portions of an insulating interlayer 102 formed over a semiconductor substrate 101, an opening d2 of a photoresist pattern 103 in an sparse area B is formed smaller than an opening d1 of the photoresist pattern 103 in an dense area A owing to the OPE. Hence, a contact hole 104b in the sparse area B is patterned relative small in diameter (cf. FIG. 2A and FIG. 2B). In doing so, the photoresist pattern 103 is formed using a general photomask consisting of a glass substrate 111 and a shield layer 112.
To suppress the OPE and to improve resolution, optical proximity correction, phase shift, and the like have been proposed. The optical proximity correction corrects diffraction and interference of light using an optical proximity correction pattern. The phase shift implements an interference effect to lower a space frequency of a pattern or to raise contrast of a specific position by making a light phase shifted appropriately using a predetermined phase shift mask.
The phase shift mask is categorized into a strong type and a weak type according to a mask manufacturing method and an amplitude of a phase shifted area. The strong type phase shift masks are divided into alternating, outrigger, and rim types. Moreover, an attenuated type phase shift mask belongs to the weak type. The attenuated type phase shift mask is called a half-tone phase shift mask capable of shifting a phase and adjusting transmitivity by a material selection.
FIG. 3 is a perspective diagram of a general halftone phase shift mask. Referring to FIG. 3, a halftone phase shift mask 300 is generally provided on a glass substrate 301. A halftone layer 302 having an opening, i.e., an exposure area, for forming a micro pattern is formed on the glass substrate 301. A shield layer 303 is formed on the halftone layer 302 having an opening bigger than that of the halftone layer 302. The shield layer 303 is generally a Cr layer. The halftone layer 302 consists of a Cr2O3 layer having a prescribed refractive index, a CrN layer having a prescribed refractive index, a MoSi layer having a prescribed refractive index, and the like. Moreover, the halftone mask 302 can be adjusted to have transmitivity of 70˜95% according to the material selection.
Meanwhile, according to high degree of semiconductor device integration and various kinds of devices provided to one chip, patterns are arranged within the device regularly or irregularly. Yet, in case that circuit patterns are complicatedly arranged, it is difficult to use the phase shift mask with a uniform reference. Namely, in forming patterns in the sparse and dense areas, respectively, OPC or a specific phase shift mask should be applied with consideration of pattern sizes and density.
However, as the definitions of sparse and dense areas are vague, it is difficult to specify a phase shift mask to be used.